In 1971, a novel compound isolated from the bark of the northwest Pacific yew tree, Taxus brevifolia Nutt. was described by Wani M. C. et al (in 1971, J.Am.Chem.Soc. 93, 2325–2327). This compound, named taxol (also known in the literature as paclitaxel) demonstrated moderate in vivo activity against the P-388, P-1534, and L-1210 murine leukaemia, the Walker 256 carcino-sarcoma, sarcoma 180, and Lewis lung tumor test systems. Taxol has a wide spectrum of anticancer activity. It has been approved by the Food and Drug Administration of United States in 1992 for the treatment of ovarian cancer and again in 1994 for the treatment of breast cancers. It has also been found to be effective against leukemia, and cancer of the head, neck, endometrium and lungs. Recently, it has also been used to treat polycystic kidney disease which accounts for ten percent of the kidney transplant among the dialysis patients (Nature p.750, 1994).
In more than twenty years since the initial report of its isolation, structure elucidation, and bioactivity, taxol has garnered support as an anticancer agent, culminating in recent FDA approval of its use against breast and ovarian cancers. There are two main reasons for the attention directed toward this drug. First, it shows promise against refractory breast and ovarian cancers, which are difficult to treat and which are responsible for the deaths of 60,000 women every year (Chemical &Engineering News 1991, (September), 11–18). Second, it exhibits a mode of action, which is unique among cancer chemotherapeutic agents. Unlike known antimicrotubule agents, which block microtubule production, taxol promotes tubulin polymerisation and stabilizes microtubules against depolymerization (Schiff P. B., et al. 1979, Nature 277, 665–667). Microtubules are important subcellular target for chemotherapeutic agents. Antimicrotubule agents, including the Vinca (Catharanthus) alkaloids, are extremely potent, requiring only a few molecules to disrupt the microtubular structure of cancer cells. The discovery of a new compound targeting these structures is of particular importance.
Despite its promise, there is a problem with taxol. This highly functionalised diterpene is isolated from inner bark of relatively rare and slow growing Pacific yew tree Taxus brevifolia, and few related species in extremely small quantities (<0.02% dry wt) (Chemical &Engineering News 1991, (September), 11–18). Himalayan yew, Taxus wallichiana Zucc. is a tree or a large shrub distributed in the north temperate zone of the Indian subcontinent (The Wealth of India (1976), A dictionary of Indian raw materials and industrial products, Vol. X, CSIR publication, New Delhi, P.132–134.). In contrast to the European yew (T. baccata Linn.), the Himalayan yew has a remarkable history of medicinal uses. The leaves of the Himalayan yew are used for treatment of hysteria, epilepsy, nervousness and as a lithic in calculus complaints while its non-poisonous fleshy arils have carminative, expectorant and stomachic properties (The Wealth of India (1976), p.132–134). In addition to the above medicinal properties, Himalayan yew (Taxus wallichiana) also contains the potent anticancer drug taxol and its important precursor 10-deacetyl baccatin III (DAB).
The currently practiced procedures for isolating taxol from bark have the disadvantages of being fatal to the source, being very difficult to carry out, and producing low yields. For example, (Vidensek et al 1990; Journal of Natural Products 53, 1609–1610) a 0.01% yield from a large-scale isolation starting with 806 lbs. or more of Taxus brevifolia bark has been reported. Similar procedures have been reported which comparably produce low yields, ranging from as low as 0.004%, up to about (but not above) 0.017%. A yield of 0.01% translates into 1 g being isolated from 10 kg of the bark, or 1 kg of taxol from 10,000 kg (approx.22,000 lbs) of the bark. A mature tree is said to yield 20–25 lbs. of bark, and this means that nearly 800–1000 trees are needed to produce a kilogram of taxol. Reported yields of taxol from various species of yew tree range from 50 mg/kg to 165 mg/kg (i.e., 0.005–0.017%). At present, bark of Taxus brevifolia is still being used as the major source of taxol. Because of (a) the low (0.01% or less) yields of taxol from the bark, (b) the relative unavailability of any other useful analogues, and (c) the need to cut the slow-growing trees to harvest the bark, it was decided that the bark was not an attractive source for taxol. Therefore, besides isolation from the bark, there are currently three avenues that are being pursued for the future production of taxol: (1) isolation from renewable plant parts, e.g., the ornamental yew clippings and needles; (2) semi-synthesis of taxol; (3) production of taxol by tissue culture procedures and (4) to find new taxanes from needles which are a renewable source of supply with anticancer properties.
The aim of present invention was to isolate different types of taxanes other than taxol, from the leaves commonly called ‘needles’ of Taxus wallichiana and evaluate their anticancer potential. In the course of these investigations a taxane brevifoliol [1] was identified which showed promising anticancer activity against in-vitro grown human cancer cell lines.